1. Field of the Invention
The invention relates to permselective, asymmetric membranes based on heat- and hydrolysis-stable aromatic polycondensates. These are particularly suitable for haemodialysis, haemodiafiltration, ultrafiltration, microfiltration and reverse osmosis and as a support membrane for the preparation of composite membranes. The invention also relates to the preparation of such membranes and to the use mentioned.
Haemodialysis, haemodiafiltration and haemofiltration are known processes for detoxification of blood (washing-out of blood), that is to say for removal of toxic metabolites present even in a low concentration and excess water. In these processes, the blood from the artery of the patient is passed through an artifical kidney in which the blood flows along a semipermeable membrane. On the other side of the membrane is a rinsing liquid of appropriate composition, into which the toxic substances migrate through the semipermeable membrane. The purified blood is passed back to the body of the patient.
In haemodialysis, transportation is effected via a solution process in the membrane, which is followed by a diffusion step, while a haemofiltration membrane is a pore membrane, the pore diameter of which determines the molecular weight exclusion limit. In haemodiafiltration, as the name of this process suggests, transportation is effected by both the principles mentioned, that is to say both separation via solution-diffusion mechanisms and separation via the membrane pore diameter.
2. Description of the Related Art
A wide range of polymers have been proposed for these processes, for example cellulose acetate (NTIS Report PB 225 069), polyacrylonitrile (German Auslegeschrift 2,145,183), polysulphone (German Auslegeschrift 2,228,537), aromatic polyamide or polyimide (German Auslegeschrift 2,342,072), diisocyanate addition polymers (German Patent Specification 3,341,847) and polyamide mixtures (EP 305,787). The membranes described in the publications mentioned all have specific disadvantages because of their chemical composition and their architectonic structure, such as a lack of strength, a lack of haemocompatibility, too high or too low an absorption of water, a lack of heat stability (important during sterilisation with superheated steam), water-soluble or blood-soluble, harmful additives, a lack of absence of defects, such as pinholes, gelatinous bodies and the like, which cause holes in the membrane after the precipitation, a lack of chemical stability or the undesirable necessity of having to blend/alloy the materials with hydrophilic polymers to achieve acceptable dialytic permeabilities.
This results in the fundamental requirement of development of new membranes for washing-out blood which offer the patient the best possible comfort with the lowest risk and lowest stress during the treatment.